I would only invite guests that I felt comfortable being naked in front of. I consider my nudity to be not as big a deal as seeing the baby come out, pooping, etc. I haven't had anyone with me during birthing other than hubby and midwives.

I wore a flannel nighty for both water births. It covered my tatas, was flowy in the belly so didn't restrict me, and was lon enough to cover most of my behind. It was perfect and didn't make me feel self-concious (I only get that "I don't care who sees me naked" thing like right as I'm pushing...then as soon as baby's out, it's gone.)

Nothing!
But I birthed at home, in my own bath, with only my husband and midwife there. I have no desire to have anyone else remotely close to me during labour! But, if you were having labour and birth pics done, or were having other family/friends around, then I know some women have chosen to wear a tankini/bikini top, with or without a swim skirt. (no proper swim bottoms, obviously.)

__________________
Me DH married for 12 years, parents to 7 small people
Five of these: and two of these: including our newest addition, born 26-08-13

I wore a sports bra that looked like a cut off tank top for both of my water births. It came off once baby was born and then I had nothing. I didn't feel it was too restrictive (of course it wasn't real tight) and I was comfortable.

Ds2 was a hospital water birth, he was born just 20 min after arriving at the hospital, I tore my clothes off so fast, they just felt so restricting.

Dd was a home birth, not a water birth and again clothing just felt cumbersome, but, I do kinda wish I had something on top because all her "fresh out" pics have boobies all up in them too so I'm self conscious of ever sharing those and hopefully she will think they are beautiful when she's grown. I'm actually kinda grateful that they are her birth and not my boys.

The next birth is a planned water birth and I think a sports bra that is not too tight or a belly band gone tube top sounds perfect, nothing on bottom.